1. Field of the Invention
The present invention relates to a convergence correction circuit and a three tube type projector including the convergence correction circuit. More particularly, the present invention relates to a convergence correction circuit which can perform high-precision convergence correction by interposing a correction filter for correcting a change in the correction frequency characteristic caused by an influence of coupling of a sub-deflecting yoke with a deflecting yoke, between a path of a correction signal supplied to the sub-deflecting yoke for convergence correction, and to a three tube type projector including the convergence correction circuit.
2. Description of the Related Art
Conventionally, there has been known a three tube type projector including projection cathode-ray tubes (projection CRT) for red, green and blue color images. As shown in FIG. 1, the tree tube type projector is constructed so that respective color images displayed on each fluorescent screen of projection cathode-ray tubes 1R, 1G and 1B for red, green and blue color images are individually projected on a screen 3 through respective projection lenses 2R, 2G and 2B, and an enlarged color image is produced on the screen 3.
In order to converge red, green and blue color images on the screen 3, a convergence correction circuit is used. More specifically, a neck portion of respective cathode-ray tubes 1R, 1G and 1B is provided with a sub-deflection yoke for convergence correction separately from the deflection yoke, and a correction signal is supplied to the sub-deflection yoke to perform convergence correction, thus red, green and blue color images are converged on the screen 3.
FIG. 2 shows a positional relationship between a main deflection yoke and a sub-deflection yoke in an electrostatic focusing (unipotential gun type) cathode-ray tube. A focusing electron lens is composed of a third grid electrode G3 functioning as an acceleration electrode, a fourth grid electrode G4 functioning as a focusing electrode and a fifth grid electrode G5 functioning as an acceleration electrode. The sub-deflection yoke 5 (horizontal sub-deflection yoke 5H, vertical sub-deflection yoke 5V) is arranged in a neck portion 6 of the cathode-ray tube and situated on the cathode side closer than the deflection yoke 4. For instance, the sub-deflection yoke 5 is arranged oppositely to the fourth and fifth grid electrodes G4 and G5.
FIG. 3 shows a configuration of a convergence correction circuit (sub-deflection output circuit) for supplying a correction current to a coil constituting a horizontal deflection yoke 5H or vertical deflection yoke 5V. As shown in FIG. 3, an input terminal 11 is led from a noninverting input terminal of an operational amplifier 12, and a convergence correction signal Scp is supplied to the input terminal 11. On the other hand, an output terminal of the operational amplifier 12 is connected to an inverting input terminal of the operational amplifier 12 through a parallel circuit which consists of a coil 13 constituting a sub-deflection yoke and a resistor 14. Also, the inverting input terminal of the operational amplifier 12 is grounded through a resistor 15. In the convergence correction circuit having a configuration as described above, when the convergence correction signal Scp is supplied to the input terminal 11, the coil 13 constituting a sub-deflection yoke is supplied with a current icp proportional to the correction signal Scp, and thus, convergence correction is performed.
In the case where the convergence correction circuit as shown in FIG. 3 supplies a current icp proportional to a correction signal Scp to the coil 13 constituting a sub-deflection yoke to carry out convergence correction, the motion of image on a screen does not coincide with a wave form of the correction signal Scp in the convergence correction; for this reason, there has arisen a problem in that high-precise convergence correction does not come true. For instance, when the correction signal Scp as shown in FIG. 4A is supplied to the input terminal 11 of the convergence circuit for a vertical sub-deflection yoke 5V, the motion of image on a screen is as indicated by a solid line "a" shown in FIG. 4B, and does not coincide with a wave form of the correction signal Scp.